When war began, London had its own ready-made shelter system - the Underground. Many Londoners used the Tube as their full-time or part-time shelter from the air raids. At first this was a little disorganized with people sleeping when and where they could, even when the trains started running again in the morning (look at the two photographs of the Elephant and Castle tube shelter). However at the peak of the Blitz, near the end of September 1940, 79 Underground stations were being used as shelters and 177,000 Londoners were sleeping in the Underground system.

The Tube was also used by government and industry. Special offices were built in disused passages and on the platforms at the Down Street, Dover Street, Hyde Park Corner, Knightsbridge, and Holborn Tube stations. Sometimes the War Cabinet met in the Tube shelters to escape the bombing. Anti Aircraft Control used another disused station as its headquarters.

In November 1940, Plessey's of Ilford put its factory underground. The factory occupied the Central Line system through Wanstead to Newbury Park, creating a production line five miles long. Mr Hugh Douglas, a foreman in Plessey's during the war, remembered the line was so long that he was given a bicycle to get around the machines. This Underground factory employed 2,000 day and night shift workers and continued to supply the RAF with aircraft parts throughout the war.
There was another Underground factory in the long, wide Underground subway that led to the Earl's Court exhibition halls. This factory also made aircraft component parts from 15th June 1942, until 14th June 1945. All of its workforce were voluntary part time transport workers.

A further eight Underground shelters were built by London Transport at particular sites that could be converted into new Underground lines after the war. The south London sites were at Clapham South, Clapham Common, Clapham North, and Stockwell. The north London sites were at Chancery Lane, Goodge Street, Camden Town, and Belsize Park. These shelters were built for the general public, with room for 64,000 people, but for most of the war they were used for military purposes. General Eisenhower used one as headquarters for co-ordinating D-day activities. However by the time the Germans started the V-weapon flying bomb attacks, five of the new shelters were opened to the public. After the war the Clapham Common shelter was the first London home for forty Jamaicans who came to Britain on the S.S. Empire Windrush.